Radon Calibrating Laboratory

The Federal Office for Radiation Protection (BfS) operates an accredited calibration service laboratory for quality assurance of radon and radon progeny measurements.

The laboratory offers calibration, type tests and comparison tests for measurement devices. Besides, it carries out scientific investigations on the metrological background, evaluation and improvement of measuring methods.

The Radon Calibration Laboratory is accredited by the German Accreditation Body (DAkkS). It maintains a quality management system and is assessed regularly.

Measurement accuracy is an important requirement for measurement devices. Via calibration the deviation of the measuring instrument indication from the correct value of the measurand is determined and documented.

In the calibration laboratory the measured value of the instrument to be tested is compared to the measured value of a very exact measurement instrument (standard). The aim is to determine the deviation from this standard and the uncertainty of measurement. Thus, the calibration of the measurement instrument can be traced back to the national primary standard via a chain of standards.

BfS Radon Calibrating Laboratory

The Radon Calibration Laboratory for devices determining the exposure to radon and radon progenies is an essential element for assuring the quality of measurements in the field of radiation protection. It is part of the BfS Coordinating Office for the Monitoring of Elevated Natural Radioactivity in Berlin.

The calibration laboratory is the recognized metrological reference for

the implementation of part 3 chapter 2 of the Radiation Protection Ordinance and

protection against elevated levels of radon exposure at workplaces and in dwellings.

Quality assurance

Part of the Radon Calibration Service Laboratory with 0.4-cubic-metre stainless-steel containers, computer-controlled measuring and dosage system

Besides the calibration of measurement devices an extensive program for quality assurance is performed to ensure a national quality standard for the measurement of radon and radon progenies. The program is focused on

manufacturers of measurement instruments to evaluate the metrological properties conformity of their measurement devices with international stated normative requirements, and

users offering a regular calibration service of measurement devices.

Interlaboratory comparisons are operated regularly for passive radon monitors. This is accompanied by proceedings to recognize the organizational, technical, and professional pre-conditions for the metrological monitoring of radon-222-exposures during work activities according to part 3 chapter 2 of the Radiation Protection Ordinance.

The offers for quality assurance of measurements of radon-222 and radon-222-progenies are used by both national and international institutions.

Technique and equipmentshow / hide

The Calibration Laboratory is accredited according to norm DIN EN ISO/IEC 17025 at the German Accreditation Body DAkkS GmbH) for the measurands activity concentration of radon-222 in air (CRn) and potential alpha-energy concentration (PAEC) of the short-lived radon-222 progenies (CP). These measurands are traced back to national primary standards at PTB.

Laboratory for the measurand potential alpha-energy concentration of the short-lived radon-222 progenies, and

Physical-technical laboratories.

Radon reference atmospheres

For the purpose of calibration of measurement devices, scientific and technical investigations radon-222 reference atmospheres are generated in several stainless steel containers with volumes of

0.4 cubic metres,

11 cubic metres ("Radon chamber") and

30 cubic metres ("PAEC chamber").

The reference atmospheres are characterized by the measurands (activity concentration of radon-222 in air and potential alpha-energy concentration of the short-lived radon-222 progenies), respectively as well as the climate parameters temperature, pressure, and relative humidity.

Temperature and relative humidity can be adjusted in both Radon chamber and PAEC chamber by corresponding technical facilities.

PAEC chamber: air turbulance and aerosol parameters

The PAEC chamber is equipped with additional technical systems for adjusting and controlling air turbulence and aerosol parameters. Aerosols with known particle size distribution and concentration can be generated.

Aerosols are subject to physical processes leading to clotting (coagulation) or the adsorption at walls, objects or floors (deposition). Considering these processes the aerosol particle concentration as well as the size distribution can be kept at a spatial and temporal sufficiently constant level within the PAEC chamber during tests.

A computer-controlled dosage system ensures a sufficient temporal stability of the radonactivity concentration. This dosage system connects the calibration containers to a flow-through source containing radium-226.

Radium-226 decays into gaseous radon-222. Thus, a flow of carrier gas like air through a container with radium-226 provides a continuous source of radon-222. This can be used to continuously compensate the natural decay of radon within the calibration chamber and the radon-222 activity concentration can be kept constant at the intended level.

Quasi-continiuous monitoring

The radon-222 activity concentration and the climate parameters are monitored quasi-continuously. Additionally, daily manual sampling is performed for each reference atmosphere to secure redundancy (at working days).

The measurement values of the radon-222 activity concentration is thus determined by two different sampling methods. The results of both methods are compared. The comparison is analyzed for quality assurance.

All measurement values obtained during the tests are recorded in data bases and can be managed centrally. Thus, the repeatability and traceability are ensured..

Measurement uncertainty of the calibration depending on the Radon-222-activity concentration range

The measurand activity concentration of radon-222 is traceable to the national standard of the German National Metrology Institute Physikalisch-Technische Bundesanstalt (PTB) via a radon-222 activity standard.

The following table shows the measurement uncertainty depending on the radon-222 activity range. The best measurement uncertainties according to the annex of the accreditation certificate is displayed.

* The extended relative measurement uncertainty is indicated, which results from the standard uncertainty of measurement multiplied with a coverage factor k = 2. It has been determined in accordance with "Guide to the Expression of Uncertainty in Measurement"(ISO, 1995). The conventional true value of the measurement quantity lies in the dedicated range of values with 95 per cent probability..

Calibration of devices measuring radon-222 progeniesshow / hide

The calibration of measurement devices for radon-222 progenies is technical more sophisticated than those of radon measurement devices. A variety of additional parameters that influence the measurement of short-lived progenies have to be accounted for and monitored. This includes:

Particle concentration and particle size distribution of the aerosol,

Relation between the aerosol-attached part and the unattached part of the short-lived progenies,

Temperature, relative humidity, and air pressure,

Flow velocity and degree of turbulence of the air in the chamber, and

Measuring position within the chamber.

For the measurement of radon progenies and the calibration of corresponding measurement devices a chamber with a large volume of 30 cubic meters is used. This provides a surface to volume ratio sufficiently small to reduce the influence of interfering parameters like adsorption to the chamber surface. Thus, the homogeneity of the reference conditions within the chamber volume is ensured.

Technical equipment

All relevant parameters are detected by sensors and logged automatically for calibration and quality assurance purposes. Furthermore, the parameters of the reference atmosphere can be adjusted and/or kept constant during the calibration process. For this purpose the following technical facilities are used:

Aerosol generators with different principles of operation and particle size distribution,

Radon progeny measurement devices are calibrated for the measurand potential alpha-energy concentration of the short-lived radon-222 progenies. This requires the determination of the activity concentrations of these progenies in the reference atmosphere, respectively.

Application of a combined mesh-filter probe

For this purpose the air of the reference atmosphere is conveyed with a constant volume flow rate through a combined mesh-filter probe. The fraction of the short-lived radon-222 progenies, which are attached to aerosols, are deposited to the filter. The free (unattached) fraction is adsorbed to the wire mesh.

The activity collected at the filter and the mesh, respectively, decays according to the radioactive decay law. The activities of the alpha-emitting nuclides are measured in periodic time intervals. The associated graphic representation is denominated as decay curve.

Immediately after sampling the decay curves of the filter and mesh alpha activity are measured, respectively. An algorithm is used to deduce the potential alpha-energy concentration and unattached part of the short-lived radon-222 progenies.

Measurement uncertainties

The measurand potential alpha-energy concentration of the short-lived radon-222 progenies is traceable to the national standard of the German National Metrology Institute Physikalisch-Technische Bundesanstalt (PTB) via a radium-226 source in filter geometry.

The following table shows the best measurement uncertainties which can be achieved during calibrations depending on the range of the measurand potential alpha-energy concentration of the short-lived radon-222 progenies according to the annex of the accreditation certificate.

Best measurement uncertainty of the measurand potential alpha-energy concentration of short-lived radon-222 progenies according to the accreditation certificate

Potential alpha-energy concentration of short-lived radon-222:
Range of the measurand

Best measurement uncertainty of the reference measurement*

* The extended relative measurement uncertainty is indicated, which is resulting from the standard uncertainty of measurement multiplied with a coverage factor k=2. It has been determined in accordance with "Guide to the Expression of Uncertainty in Measurement" (ISO, 1995). The conventional true value of the measurement quantity lies in the dedicated range of values with 95 per cent probability.

Schmidt, V., Hamel, P.
Measurements of Deposition Velocity of Radon Decay Products for Examination of the Correlation between Air Activity Concentration of Radon and the accumulated Po-210-Activity, The Science of the Total Environment, 272 (2001), S. 189-194

Improvement of methods

The quality assurance program ensures a constant striving for the further development of methods. Scientific investigations are performed in regard to metrological basics as well as to the evaluation and development of measurement methods. This is to provide the ability to act promptly on current issues.

In this context, an innovative measurement system for the determination of radon exhalation from building materials was developed and corresponding measurements were performed. This takes account of the increasing public interest in reliable information on natural radiation exposure in people’s living environment.

the potential alpha energy concentration of the short-lived radon-222 progenies (polonium-218; lead-214; bismuth-214) in air (Cp).

The measurands are traced back to the national standards at the Physikalisch-TechnischeBundesanstalt (PTB). The table „Measurands, measurand ranges and best measurement uncertainty for calibrations“ shows the respective uncertainty budgets.

For calibration the measurement instrument is exposed to a reference atmosphere with defined values of the measurand as well as potentially influencing factors. The indication of the calibration object is compared to the reference value. The result of the calibration is given as the respective ratio of these values.

Measureands, measureand ranges and best measurement uncertainty for calibrations

* The extended relative measurement uncertainty is indicated, which is resulting from the standard uncertainty of measurement multiplied with a coverage factor k=2. It has been determined in accordance with "Guide to the Expression of Uncertainty in Measurement" (ISO, 1995). The conventional true value of the measurement quantity lies in the dedicated range of values with 95 per cent probability.

The costs for calibration services will be charged according to the time involved. An estimate will be communicated in advance upon request.

The measurand to be calibrated is the exposition PRef. The exposition is the product of the mean radon-222 activity concentration CRef and the exposure time tExp.

For calibration the measurement systems are exposed to a reference atmosphere within a container for a certain time. Exposure time and mean radonactivity concentration are chosen to match the required exposition. The radon-222activity concentration is determined using a working standard. During exposition it is kept constant as far as possible within a total range of 0.4 to 12 kilobecquerels per cubic metre.

measurement range: 30 to 20,000 kilobecquerels per cubic metre multiplied by the number of hours

best measurement uncertainty of the reference measurement (2σ, standard uncertainty multiplied with a coverage factor k=2): ± 7 per cent

The costs for calibration expositions will be charged according to the time involved. An estimate will be communicated in advance upon request.

Testing of the metrological properties of measurement systemsshow / hide

Investigations of radon-222 and radon-222 progeny measurement system properties as well as the influence of external parameters on the displayed values will be conducted upon request. Costs will be charged according to the time involved.

Testing of measurement methods show / hide

Methods for the measurement of radon-222 and radon-222 progenies will be tested upon request. Costs will be charged according to the time involved.